1. Field of the Invention
The present invention disclosed in the specification relates to a method for producing a crystalline silicon film on a substrate having an insulating surface such as a glass substrate.
2. Description of the Related Art
A technique for constructing a thin film transistor (TFT) using a silicon thin film formed on a glass substrate has been concerned. The TFT is utilized mainly in an active matrix type liquid crystal electro-optical device and also in various thin film integrated circuits. In the liquid crystal electro-optical device, a liquid crystal is injected between a pair of glass substrates. An electric field is applied to the liquid crystal, to change an optical characteristic of the liquid crystal is changed and image display is performed.
In particular, the active matrix type liquid crystal display device is characterized in that a TFT is disposed for each pixel and charges stored in a pixel electrode are controlled by using the TFT as a switch. Since the active matrix type liquid crystal display device can display a fine (high resolution) image at a high speed, it is utilized for a display device of various electronic devices such as a portable word processor and a portable computer.
In general, an amorphous silicon thin film is used for a TFT utilized in the active matrix type liquid crystal display device. However, in the TFT using an amorphous silicon thin film, there are problems that (1) since its characteristic is low, a high quality image cannot be displayed and (2) a peripheral circuit for driving TFTs disposed in pixels cannot be constructed.
It can be considered that the above problem (2) includes two problems. That is, one is that since a P-channel type TFT cannot be formed in practice using an amorphous silicon thin film, a complementary metal-oxide semiconductor (CMOS) circuit cannot be constructed, and the other is that since the TFT using an amorphous silicon thin film cannot be operated at a high speed and a large amount of electric current cannot be supplied, a peripheral driver circuit cannot be constructed.
As a method to solve the above problems, there is a technique for forming a TFT using a crystalline silicon thin film. As a method for obtaining the crystalline silicon thin film, there are a method for thermally processing an amorphous silicon film and a method for irradiating an amorphous silicon thin film with a laser light.
A method for crystallizing an amorphous silicon film by thermal processing includes in general a problem described below. When a TFT utilized in a liquid crystal electro-optical device is constructed, it is necessary to form the TFT on a transparent substrate. As the transparent substrate, there are a quartz substrate and a glass substrate. However, since the quartz substrate is expensive, it cannot be utilized in the liquid crystal electro-optical device where cost reduction is an important technical problem. Thus, a glass substrate is utilized generally. However, there is a problem that a heat resistance temperature of the glass substrate is low.
In general, Corning 7059 glass substrate used as a glass substrate utilized in a liquid crystal electro-optical device has a distortion point of 593.degree. C. When it is heated at a temperature no lower than this temperature (the distortion point), the substrate is distorted markedly. Also, Corning 1737 glass substrate is known as a glass substrate which can be heated at a high temperature, (a distortion point is 667.degree. C.). However, even though this glass substrate is used, when its area is enlarged to a large area, it is not desired to perform heating treatment for 10 hours or longer at 600.degree. C. or higher.
In particular, since a liquid crystal electro-optical device tends to make a large area, it is necessary to suppress shrinking and distortion of the substrate. To solve this problem, it is desired that a crystallization temperature is set to be about 550.degree. C. or lower and a heating time is several hours or shorter.
Further, a technique for crystallizing an amorphous silicon film by laser light irradiation is also known. This method has the advantage that it is not necessary to perform heating treatment at a temperature that a problem occurs. However, there is a problem that it is difficult to obtain an effect by laser light irradiation at high repeatability.